The invention relates to a new catalyst and a method using that catalyst in reforming hydrocarbons, more particularly hydrocarbons comprising paraffins containing at least 6 carbon atoms, to form the corresponding aromatic hydrocarbons.
Catalytic reforming is well known in the petroleum industry and refers to the treatment of naphtha fractions to improve the octane rating. The more important hydrocarbon reactions occurring during reforming operation include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, and dehydrocyclization of paraffins to aromatics. Hydrocracking reactions which produce high yields of light gaseous hydrocarbons, e.g., methane, ethane, propane and butane are to be particularly minimized during reforming as this decreases the yield of gasoline boiling products.
Dehydrocyclization is one of the main reactions in the reforming process. The conventional methods of performing these dehydrocyclization reactions are based on the use of catalysts comprising a noble metal on a carrier. Known catalysts of this kind are based on alumina carrying 0.2% to 0.8% by weight of platinum and preferably a second auxiliary metal.
The possibility of using carriers other than alumina has also been studied and it was proposed to use certain molecular sieves such as X and Y zeolites, which appeared suitable provided that the reactant and product molecules were sufficiently small to pass through the pores of the zeolite. However, catalysts based upon these molecular sieves have not been commercially successful.
In the conventional method of carrying out the aforementioned dehydrocyclization, paraffins to be converted are passed over the catalyst, in the presence of hydrogen, at temperatures of the order of 500.degree. C. and pressures of from 5 to 30 bars. Part of the paraffins are converted into aromatic hydrocarbons, and the reaction is accompanied by isomerization and cracking reactions which also convert the paraffins into isoparaffins and lighter hydrocarbons.
The rate of conversion of the hydrocarbons into aromatic hydrocarbons varies with the reaction conditions and the nature of the catalyst.
The catalysts hitherto used have given moderately satisfactory results with heavy paraffins, but less satisfactory results with C.sub.6 -C.sub.8 paraffins, particularly C.sub.6 paraffins. Catalysts based on a zeolite of the L family are more selective with regard to the dehydrocyclization reaction; can be used to improve the rate of conversion to aromatic hydrocarbons without requiring higher temperatures, which usually have a considerable adverse effect on the stability of the catalyst; and produce excellent results with C.sub.6 -C.sub.8 paraffins, but stability is a problem.
In one method of dehydrocyclizing aliphatic hydrocarbons, hydrocarbons are contacted in the presence of hydrogen with a catalyst consisting essentially of a type L zeolite having exchangeable cations of which at least 90% are alkali metal ions selected from the group consisting of ions of sodium, lithium, potassium, rubidium and cesium and containing at least one metal selected from the group which consists of metals of Group VIII of the Periodic Table of Elements, tin and germanium, said metal or metals including at least one metal from Group VIII of said Periodic Table having a dehydrogenating effect, so as to convert at least part of the feedstock into aromatic hydrocarbons.
A particularly advantageous embodiment of this method is a platinum/alkali metal/type L zeolite catalyst because of its excellent activity and selectivity for converting hexanes and heptanes to aromatics, but stability remains a problem.